Improve `String.codePointAt` to use the native method in ES2015+.
Improve `String.codePointAt` to use the native method in ES2015+.

scala-js/scala-js,sjrd/scala-js,gzm0/scala-js,sjrd/scala-js,sjrd/scala-js,scala-js/scala-js,gzm0/scala-js,scala-js/scala-js